1. Field of the Invention
The present invention relates to a phenol novolak resin which is useful as a film-forming component of a photosensitive composition, to a process for synthetically producing the resin, and to a positive photoresist composition using the resin.
2. Description of the Related Art
Phenol novolak resins obtained by condensation reaction of phenols such as cresol, xylenol, and trimethylphenol are generally used as film-forming components of positive photoresist compositions.
As a positive photoresist composition for use in i-line (365 nm) lithography, a positive photoresist composition including the phenol novolak resin and a 1,2-naphthoquinonediazide-group-containing compound is excellent in sensitivity, definition, and heat resistance and is widely practically used in the fields of the manufacture of semiconductor integrated circuits (ICs).
The density of such ICs becomes denser and denser with advance of semiconductor technologies, and particularly in the production of logic ICs, demands have been made on photoresist materials that can form a fine resist pattern of not more than 0.35 μm with a good shape both in a dense pattern or in an isolation pattern.
However, conventional positive photoresist compositions have strong tendency of reduction in film thickness and are believed not to form a fine pattern of not more than 0.35 μm with a good shape. Specifically, the pattern is tapered to thereby invite twist of pattern formation in an isolation pattern, and a separated pattern cannot be significantly formed or the film thickness is significantly reduced in a dense pattern.
A phenol novolak resin having a p-cresol constitutional unit or 3,4-xylenol constitutional unit is believed to be suitable for the preparation of a high definition positive photoresist composition. However, a positive photoresist composition using a phenol novolak resin containing p-cresol constitutional unit cannot provide satisfactory sensitivity compatible with high definition, and a positive photoresist composition using a phenol novolak resin containing 3,4-xylenol constitutional unit cannot form a satisfactory isolation pattern and the resulting isolation pattern is low in focal depth range properties, although the composition is excellent in sensitivity and definition.
Separately, demands have been made on means to produce IC chips at low cost with decreasing prices of IC chips, in order to make profits.
Especially, the production of highly integrated logic ICs including mixtures of dense patterns and isolation patterns of not more than 0.35 μm requires a large number of production steps and invites the cost to increase. To minimize the number of such production steps, demands are made on resist materials that can concurrently form such fine dense patterns and isolation patterns.
A variety of novolak resins containing xylenol constitutional unit or trimethylphenol constitutional unit have been proposed as such resist materials, but even these novolak resins cannot form dense patterns and isolation patterns with good shapes without deterioration in sensitivity and definition, in the formation of fine resist patterns of not more than 0.35 μm.
As thus described, in the formation where a definition of not more than 0.35 μm is required, the focal depth range properties are markedly deteriorated and resist patterns that can be used in practice cannot be formed. Therefore, demands are also made on materials that can form both dense patterns and isolation patterns with good shapes and have satisfactory focal depth range properties in the formation of fine resist patterns of not more than 0.35 μm.
Phenol novolak resins synthetically produced from two or more types of phenols are generally used in the positive photoresist composition. Individual phenols have, however, different reactivity. For example, p-cresol has a poor reactivity and most of p-cresol used as a starting material provides oligo-nuclear products.
Generally, part or overall of low-molecular-weight fractions having a molecular weight of not more than 2000, such as oligo-nuclear products of p-cresol, of phenol novolak resins used in the positive photoresist composition are removed by fractionation operation. Accordingly, if the amount of such oligo-nuclear products is large, the yield of the resulting phenol novolak resin is decreased and the production cost of the positive photoresist composition is increased.
Another type of phenol novolak resins is synthetically produced by using 3,5-xylenol, m-cresol, 3,4-xylenol, 2,5-xylenol, and other phenols having a high reactivity. This phenol novolak resin may have different resin composition [proportions of individual constitutional units or the ratio of ortho-ortho bond(o-o)/ortho-para bond (o-p)/para-para bond (p-p)] in high molecular weight fractions and in low molecular weight fractions of the resin, and the fractionation operation invites great variations in resin characteristics. The phenol novolak resin cannot therefore significantly provide a positive photoresist composition having stable characteristics.
Under these circumstance, demands have been made on a process for synthetically producing a phenol novolak resin that can inhibit the formation of oligo-nuclear products of phenols, where the resulting phenol novolak resin has no difference in resin composition both in high molecular weight fractions and low molecular weight fractions.